With My Computer, open the Windows directory and locate notepad.exe. Right click - copy.
Then Back to "programs" directory and create a folder named "notepad". Then open that and right-click, paste notepad.exe into folder.
Close folder. Next, find a .dat file and right click on it, opening the menu where you can select the program to open .dat files. click browse, go to Notepad folder, open and click on "notepad.exe." Suddenly a whole list of extensions appears, as if by magic. Reclick on notepad.exe again and click "always open with...".
This was the first thing in Vista I have encountered that made me angry with MS.It used to be so easy to do this. I'm really p.o.'d at Nero for not creating a filter for the dat (sound) file, rather than allow itself to select all .dat files. I hope this helped, if you still needed it. I couldn't figure out anything from any of the other posts. I had to go work it out myself. People seem too excited by seeing their words in print these days, or gossiping with each other in help columns.
I guess, or else one of the "experts" would have passed the real fix along to you sooner.
D.Allen